19 March 2022 | New Scientist | 15SOME of the moon’s craters may
contain “double-shadowed”
regions that are so dark they
would be among the coldest
places in the solar system.
The small tilt of the moon –
just 1.5 degrees – as it orbits with
Earth around the sun means
that it has hundreds of craters
where direct sunlight never
reaches. We know that inside
these craters, located near the
moon’s poles, temperatures
can drop below -170°C, making
them prime locations for water
ice to collect and optimum
locations for future human
missions, as astronauts could
use the ice as a source of water
for their missions.
Even though the insides of
these craters don’t receive direct
sunlight, they can be heated
by sunlight reflecting off their
rims, which can melt some of
their more exotic ices, such as
carbon dioxide ice.
Now, Patrick O’Brien and
Shane Byrne at the University
of Arizona in Tucson think
they have found even darker
craters that are shielded from
this reflected sunlight. These
double-shadowed regions
would be rare, a fraction of
a per cent of the total area of
craters that don’t receive directsunlight, with temperatures
dropping to -250°C.
“Their main source of light
is starlight,” says O’Brien, who
presented the work at the
Lunar and Planetary Science
Conference in Texas on 7 March.
“They could be the coldest
places in the solar system.”
Evidence for these frigid
craters comes from NASA’s
Lunar Reconnaissance Orbiter,which uses a laser to study
the moon’s surface. Billions
of pulses have been fired at the
moon for more than a decade,
says O’Brien, allowing detailed
measurements of the lunar
surface to be made. The team
used this information to search
for and examine these double-
shadowed craters.
“They’ve been predicted, but
we’re the first to actually look
for them on the moon,” he says.
For a double-shadowed crater
to exist, it must be sufficiently
deep and set at an angle that
wouldn’t allow sunlight to
be reflected in. In total, the
researchers found hundredsof suitable craters that could
host double-shadowed regions,
ranging in size from 100 to
600 metres across, but the team
says there could be many more
smaller ones, with diameters
of just tens of centimetres.
Margaret Landis at the
University of Colorado Boulder,
who wasn’t involved in the
research, says these colder
regions could help us work out
where water ice or other exotic
ices on the moon and other
bodies came from.
Temperatures of -170°C
“are great to preserve water
ice, but too warm for other
ices like carbon dioxide, or
organic species that might
be a fingerprint of a comet
impact”, she says. Such impacts
could have been a source of
Earth’s water.
“It has massive implications
for the amount of water that
Earth got from non-Earth
sources,” says Landis.
An upcoming NASA lunar
rover may be able to drive into
some of these regions. Called
VIPER, it is scheduled to arrive
at the moon’s south pole in
November 2023 and will drive
for up to 10 hours at a time
into three regions that never
receive direct sunlight. It will
use a drill and headlights to
look for ice, and it may also
discover some of these double-
shadowed craters.
“One of our objectives
is to locate and observe
multi-shadowed craters,”
says Anthony Colaprete at
NASA’s Ames Research Center,
the VIPER mission’s lead
scientist. “It’s going to be
pretty awesome.” ❚Shackleton crater
sits at the moon’s
south poleAstronomyJonathan O’CallaghanJORGEMAÑE
S^ RUBIO/ES
A/ACT
/DITISHOE-250 ̊C
Temperature in lunar craters
that see no reflected lightDouble-shadowed craters
could hold ice on the moon
COMPUTER simulations that
trace how our moon formed in
high resolution may explain the
mystery of why it is so chemically
similar to Earth.
The conventional story for the
moon’s origin is that a primordial
planet named Theia smashed into
Earth and spewed molten rock into
space. This debris, primarily made
up of Theia, then coalesced into
the moon over a period of tens
of millions of years.
While this scenario accounts
for the moon’s observed angular
momentum, it fails to explain the
near-identical profile of its isotopes
to those found on Earth. Isotopes
are atoms of the same element that
differ by the amount of neutrons
they contain, and the ratio of
different isotopes in a sample
can be used to trace its origin.
Jacob Kegerreis at Durham
University in the UK and his
colleagues ran more than 400
high-resolution simulations of
what might have happened when
the early Earth was struck, using
different initial conditions like
impact angle and speed.
Many of the simulations
showed a satellite forming within
hours of the impact – much more
quickly than suggested by previous
research. They produced a moon
with a similar angular momentum
and isotopic make-up to Earth,
which means that the moon
contains more of Earth and less of
Theia than other models supposed.
The findings were presented at
the Lunar and Planetary Science
Conference in Texas on 10 March.
“It’s interesting that simulations
in this work lead to more mixing
between the impactor and
proto-Earth than previous work
suggested,” says Miki Nakajima
at the University of Rochester in
New York. “This would help explain
the isotopic similarities between
Earth and the moon.” ❚AstrophysicsAlex WilkinsMoon’s emergence
from planet crash
reconstructed